1. Field of the Invention
The invention relates generally to the field of subsurface rock formation productivity estimation. More specifically, the invention relates to methods for determining expected fluid production from nuclear magnetic resonance measurements.
2. Background Art
Wellbores are drilled through subsurface rock formations to extract useful substances such as oil and gas. A wellbore forms a hydraulic conduit from a permeable subsurface rock formation having oil and/or gas present therein to the Earth's surface. Oil and/or gas typically move to the surface through the wellbore by the force of gravity. Gravity manifests itself as a pressure drop between the fluid pressure in the pore spaces of the subsurface rock formation and the wellbore. The rate at which the oil and/or gas move into the wellbore and to the surface depends on the pressure drop between the formation and the wellbore, the viscosity of the oil and/or gas, and the permeability of the rock formation to the oil and/or gas.
The relative volumes of gas, water and oil that flow from a particular rock formation depend not only on the pressure differential between the formation and the wellbore, but also on the relative permeability of the rock formation to each of the respective fluids and the viscosity of the fluids. In combination, the viscosity and relative permeability define a property of a fluid in a permeable formation known as the fluid mobility. Because of capillary pressure effects, mobility of a particular fluid is also dependent on the fractional volume of the pore space occupied by the fluid and on the pore structure of the pore space in the particular rock formation. It is desirable to be able to determine fluid mobility and likely fluid production rates of gas, oil and/or water from subsurface rock formations in order to estimate the economic value of the rock formation for its possible hydrocarbon production.
It is known in the art to estimate productivity by various forms of fluid sample extraction and production testing. The foregoing productivity estimating techniques require extensive use of equipment, even installation of pipe or casing into the wellbore for accurate testing. There exists a need for productivity estimation using non-invasive, quick and convenient well logging techniques,